This invention relates generally to color printing and, more specifically, to a method and the apparatus for employing the method of continuously superimposing a plurality of color toner images onto a common intermediate transfer belt from a plurality of developing drums. The latent image is developed with liquid toner.
Prior electrostatic copiers or printers employing liquid toners transfer the toned image from a photoreceptor or a master directly to the receiving paper substrate. These devices require that the solvent or liquid in which the toner particles are suspended be transferred to the receiving paper substrate. This requires drying of the paper before it can be used and adds to the complexity of the devices.
Other copiers and printers employ an intermediate transfer belt or drum to transfer the toned image to paper by heat and pressure. These prior systems have required the receiving paper substrate to be registered to each of the plurality of colors, adding to the complexity of the apparatus.
A system employing a liquid toner has been developed to transfer a liquid developed image from a photoconductor to a copy sheet via an intermediate transfer surface from which the carrier liquid is roller squeezed or removed by infrared heating to be substantially free of carrier liquid prior to the final image transfer to the copy sheet. However, this does not remove all of the solvent from the copy sheet, since solvent is still present in the image areas. The intermediate transfer surface is formed from a material described as non-absorbing and resilient, but transfer from the photoconductor to the intermediate transfer surface is effected by contact pressure and the intermediate transfer surface is deformed by contact with the toner particles in the image areas to achieve the transfer from the photoconductor covered drum to the intermediate transfer surface. This negatively affects the quality of the transferred image by distorting the image because of the contact or pressure involved in the transfer step.
A number of the prior approaches utilized in electrophotographic copiers have employed dry powder toner that was contact or pressure transferred from the photoconductive surface to an intermediate transfer surface and then to the final receiving surface. These approaches were also susceptible to image distortion during the transfer from the photoconductor because of the pressure or contact involved in the transfer step. They also transferred less than 100% of the toner particles from the intermediate transfer surface to the final receiving surface. None of these approaches attempted to use a liquid toner to improve the resolution of the transferred image.
One such system utilized an electrophotographic copier with a rotatable photoconductive drum that transferred a dry toner developed image to a silicone elastomer transfer belt that was part of a transfer and fusing system. This was employed in combination with a radiant fuser and paper transport system to provide a high speed copier.
Another related system employed an intermediate transfer drum which received the dry toner developed image from a rotatable drum whose surface was coated with a photoconductor. The intermediate transfer drum utilized a support material, such as aluminum, and had its surface coated with a suitable conductive or non-conductive silicone rubber having low specific heat that was applied in a thin layer. These intermediate transfer surfaces were described as having smooth surfaces of low surface free energy and a hardness of from 3 to 70 durometers.
Compositions designed specifically for use as thermally conductive elastomers in a fuser roller for electrostatic copying machines were developed by the Dow Corning Corporation. The compositions were thermally conductive polyorganosiloxane elastomers that possessed high abrasion resistance, low durometer hardness and high heat conductivity.
Xerox Corporation developed an elastomeric intermediate transfer surface that was either formed into a belt or was formed on the surface of a drum as part of a process to transfer a dry powder xerographic image from a photoconductive surface to a final support surface, such as paper. Heat and pressure were utilized to transfer the developed powder image from the intermediate elastomeric transfer surface to the paper. However, this and all of the previously described approaches suffered from the aforementioned defects of image distortion and less than 100% toner particle transfer.
These problems are solved in the transfer method of the present invention and in the design of the electrostatic color printing system utilizing an intermediate image transfer belt where a plurality of toned color images corresponding to separate color separations are continuously individually superimposed onto a common intermediate transfer belt from a plurality of developing stations through a liquid-filled gap.